The present invention relates to high temperature superconducting materials and methods for their fabrication. More particularly, the present invention relates to high temperature superconducting materials fabricated in such a manner as to be substantially free from contaminants. 2. Background of the Invention
It is now generally recognized that superconducting materials hold a great potential for the advancement of technology. For example, in remarks delivered on July 28, 1987 to the Federal Conference on Commercial Application of Semiconductivity, the President of the United States stated that "to most of us laymen, superconductivity was a completely new term, but it wasn't long before we learned of the great promise it held out to alter our world for the better--a quantum leap in energy efficiency that would bring with it a host of benefits, not least among them a reduced dependence on foreign oil, a cleaner environment, and a stronger national economy."
The phenomenon of superconductivity has been known since the early part of the twentieth century. Superconductivity is generally defined as a condition which exists when a material meets two tests. These tests include (1) zero resistance, and (2) the so-called Meissner effect which comprises the repulsion (2) of a magnetic field much as one magnet repels another. When these two tests are met, a material is classified as superconducting and exhibits some unique properties, most notably zero resistance as mentioned above. It has generally been observed, however, that superconductivity occurs only at very low temperatures. The onset of superconductivity was typically not observed until temperatures dropped to the range of approximately 23K or below.
It is apparent that a material having zero resistance has many important applications. For example, it may be possible to provide superconducting electric transmission lines which would be able to carry electricity without significant loss of electrical power. This would result in the saving of billions of dollars in transmission costs and would allow the development of energy generating facilities, such as nuclear power plants, far from urban centers.
It has also been pointed out that superconductivity may allow motors to be produced which are one-tenth of the normal size. Those knowledgeable of the art have also speculated that superconductive materials would allow the production of high speed trains levitated by magnets, as well as the production of computers which would be smaller and much faster than those presently known. It has also been predicted that new superconductive data transmission lines could be constructed that would carry one trillion bits of information per second, which is approximately 100 times faster than the fiberoptic cables that carry many data transmissions and phone calls at the present time.
The use of superconductivity in these types of devices is not entirely speculative. For example, it has now been reported that the Argonne National Laboratory constructed a superconducting electric motor during late 1987. In addition, certain types of superconducting materials have been known and employed for some time. Niobium superconducting products have been marketed and have been used in various contexts, such as superconducting magnets for research purposes and in certain types of medical equipment.
One of the primary limitations on the development of superconducting products, such as motors, transmission lines, and the like, has been the fact that most superconducting materials must operate only at extremely low temperatures of approximately 4K. In order to produce these extremely low temperatures it is necessary to employ very expensive liquid helium as a refrigerant. Clearly, the extremely low temperatures at which known superconducting materials must operate severely limits their usefulness in practical everyday applications.
Recently, it has been observed that certain types of materials, particularly ceramics, exhibit superconductive properties at temperatures significantly above those of traditional superconducting materials. For example, a ceramic made of lanthanum-barium-copper-oxide has been found to be superconducting at approximately 30K. Indeed, recent developments have shown that other types of ceramic material can be superconducting at the temperature of liquid nitrogen (about 77K or higher). The production of liquid nitrogen is relatively inexpensive and its use is relatively simple. Such materials are often referred to as "high temperature" superconducting materials. That is, they exhibit superconducting properties at temperatures obtainable using liquid nitrogen.
In approximately early 1986, a compound having the general chemical formula Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y) was found to show the features of onset of superconductivity near 30K. Subsequently, findings have been reported with a compound comprised of yttrium, barium, copper and oxygen exhibiting superconductivity in the 80K to 93K range, temperatures well within the range obtainable using liquid nitrogen.
Due to the increase in temperature at which superconductivity is found to exist, it has been theorized that superconductivity ultimately may be found near room temperature. It is apparent, however, that the technique for fabricating such superconducting materials will be an extremely important factor in increasing the temperature at which superconductivity is exhibited.
Superconducting ceramics are often produced by mixing powders together to form a crystalline substance having superconductive properties at low temperature. However, such mixing produces a material having a crystalline structure which is predetermined by the crystalline structure within the powders. It is very difficult using solid mixing techniques to produce mixing at a molecular level because long diffusion times are required to achieve a homogeneous solid.
Certain other types of difficult processes have been developed for the production of superconducting materials. One such process involves the vapor deposition of materials to form a superconducting composition. Vapor deposition is complex because it requires the vaporization of materials that normally exist in a solid state. This procedure requires expensive equipment and highly technical procedures.
It has also been found that superconducting materials can be produced by a coprecipitation process. Generally, the material is dissolved in an organic solvent. Conditions are then adjusted so that a superconducting material will precipitate out of the solution. It is found, however, that the resulting precipitate includes a significant amount of the organic solvent and other chemical species that happen to be present in solution. The presence of carbon-containing contaminates can cause the formation of BaCO.sub.3 and other impurities. These and other dissolved molecules may severely disrupt the resulting crystalline structure, either of which will prevent the material from becoming superconductive, cause the material to be superconductive only at very low temperatures, or prevent the material from carrying commercially useful currents While superconductive materials can be formed which contain these materials, it is better if they are not present.
Another problem in coprecipitation processes is the control of pH. In order to maintain the pH of the solution within desirable ranges which produce desirable products, it is generally necessary to add acids or bases to control pH. For example, citric acid and oxylates are often added to the solution. When the superconducting precipitate forms it contains traces of the acids or bases used to control the pH. These substances may also form compounds with these acids or bases which can inhibit the formation of desired superconducting material.
A particular problem is encountered if ammonium hydroxide is added to control pH. The ammonium hydroxide is known to form a complex with copper in solution. One technique used to prevent the copper complex formation is to treat the precipitate with lithium. The treatment with lithium, however, itself adds a trace of lithium to the precipitate. Therefore, it is then necessary to attempt to remove the lithium from the precipitate. Thus, the addition of any unnecessary component to the solution during precipitation greatly increases the complexity of the process.
In summary, it can be seen that any type of material, other than that needed to form the precipitate, will pollute or contaminate the resulting product. Organic solvents and the like are found to become incorporated within the resulting superconductive material Likewise, acids and bases needed to control pH are also found in the resulting precipitate. These pollutants disturb the crystalline structure of the superconductive material.
In order to produce an acceptable superconductor it is necessary for it to have high electrical current capability. This is also produced by a regular grain-to-grain crystalline contact of less than the coherence length (approximately 15 Angstroms in ceramic superconductors). If such a grain-to-grain structure does not exist, the material becomes electrically resistant. It will be appreciated from the discussion above that one way to minimize the grain-to-grain contact distance of polycrystalline materials is to minimize the contaminants in the material. Contaminants are usually found at the grain boundaries and will thus increase the grain-to-grain contact distance.
Accordingly, it would be a major advancement in the art to provide high temperature superconducting materials which were extremely high in purity. It would be another advancement in the art to produce such high purity superconducting materials using solution chemistry in order to avoid the difficulties encountered in producing superconductors by other techniques. It would be a further advancement in the art to provide such superconducting materials using solution chemistry without the necessity of adding contaminants in the form of acids, bases or organic materials into the solution. Specifically, it would be an advancement in the art to use water as a solvent and to control the pH without adding species which do not form a part of the superconducting material.
Such methods and compositions are disclosed and claimed herein.